1. Field of the Invention
The present invention relates generally to polishing processes and, more particularly, to a polishing process for refractory materials, such as natural and synthetic (CVD) diamond films, gems of sapphire or ruby, silicon nitride balls and the like.
2. The Prior Art
Refractory materials excel in many desirable properties. These include high hardness, high melting point, being heat resistant and chemically inert, high thermal conductivity, high electrical resistivity, and some of them also possessing optical transparency. These desirable qualities suggest a multitude of uses and applications. Yet the widespread use of these refractory materials has been hampered, inter alia, by, their rarity, their cost and most importantly, difficulty in shaping and treating these materials, in particular, polishing their surfaces to the requirements of optical applications.
Potential uses and applications of such refractory materials include bearing and wear resistant surfaces, heat sinks, optical castings, optical windows, and also as active electronic device elements. As early as 1955, some workers heralded a process for synthetic diamond at relatively high pressures where diamond is thermodynamically stable. See F. P. Bundy et al, Nature 176 (1955), p. 51. A more cost-effective process for producing synthetic diamond at low pressure has occurred with the development of chemical vapor deposition (CVD). See J. C. Angus et al, "Diamond Growth At Low Pressures," MRS Bulletin, pp. 38-47, Oct. 1989. The CVD process for producing thin synthetic CVD diamond films inexpensively and en masse has revolutionized the diamond industry. The CVD diamond films are pure but are polyhigh polycrystalline, hence optically rough, possessin an extremely high absorption coefficient of about 150 cm.sup.-1 due to scattering at the rough surfaces. Scattering from these rough surfaces in turn results in a low transmittance, especially in the visible and the infrared (IR) regions. Consequently, the optical rough surfaces of CVD diamond films inhibit their use as a lens coating material for IR or X-ray optical windows. See X. H. Wang et al, "Infrared Optical Properties of CVD Diamond Films," J. Mater. Res., Vol. 5, No. 11, Nov. 1990. Some workers in the field have attempted to remedy the optical roughness of CVD diamond films by lowering the deposition pressure or increasing the CH.sub.4 concentrations in an effort to reduce the size of the deposited diamond crystal grains. See C. J. H. Wost et al., "Optical and Other Properties of MPACVD Diamond," Proc. SPIE, 1112, pp. 199-204 (1989). Such remedies run the risk of altering the chemical composition of the resultant CVD diamond films and/or changing certain of their physical properties.
The only other known way of reducing scatter is to polish the CVD deposited diamond films. Since however, diamond is the hardest substance known, meriting a 10 on the Mohs scale and also is chemically inert, its surface polishing is no mean task. As has been reported, CVD diamond films have been polished, with the films deposited on silicon using a cast iron lap and various mesh sizes of natural diamond polishing compounds. After many hours of polishing, a reasonable surface finish has been achieved, with optical transmission approaching that of single crystal diamond. However, the reporter in his conclusion states that further improvements in the CVD diamond surface finish are required to make quantitive measurements of the bulk optical absorption of such CVD diamond films. See Peter Taborek, "Optical Properties of Microcrystalline CVD Diamond," Proc. SPIE, 1112, pp. 205-211 (1989). Another polishing process which has been reported took six weeks of polishing CVD diamond films. Therein, the rough surface of the diamond film is rubbed against a cast-iron block heated to about 350.degree. C. After six weeks of polishing, the surface was very smooth indeed. Ion beam machining of diamond tools and knives also have surfaced in the literature. See I. T. Miyamoto et al, "Ion Beam Machining of Single-Point Diamond Tools for Nano-Precision Turning," Nanotechnology, Vol. 1, No. 1, pp. 44-49, Tokyo (1990); I. K. Miyamoto et al., "Sharpening Diamond Knives Having a Small Apex Angle of Less than 55 Degrees with High-Energy Argon Ion Beams," J. Mater. Sci. Lett., Vol. 7, No. 11, pp. 1175-1177, London (1988); and E. G. Spencer et al., "Ion Machining of Diamond," J. Appl. Phys., Vol. 43, No. 7, pp. 2956-2958 (1972). As far as known however, there are now no polishing processes reported that reliably and economically produce an optically required smooth surface on CVD produced diamond films.